1. Field of the Invention
The present invention describes a method for the coating of a saw wire with a metal or a metal alloy in which hard materials are embedded.
2. Background Art
The coating of wires for sawing operations is generally carried out in such a manner that a metal is applied to the wire by chemical or electrodeposition means and simultaneously, abrasive particles of high hardness required for sawing are embedded in this metal. A method of this type and the advantageous properties of a wire which has been coated in this way are described in published PCT application WO 01/85376, published on Nov. 15, 2001, and its European equivalent, EP 128 06 29 A1. Wires of this type are used to saw materials which are used, for example, in the semiconductor and optical industry.
Sawing with a coated wire of this type is preferably carried out on multiwire saws which require wire lengths of greater than 1000 m for operation. Hitherto, the coating of a wire by electrodeposition has been carried out in a continuous process. The wire runs from a stock reel and is fed into the coating apparatus. The coating apparatus comprises linearly arranged bath vessels which contain the various pretreatment chemicals, rinsing water and finally the coating bath. This design of the apparatus enables the wire to be guided through the individual baths over a large number of guide rolls. The guide rolls are inevitably installed in bath vessels which contain aggressive liquid media. This has the disadvantageous consequence that the bearings of the guide rolls rapidly become worn. After the pretreatment has ended, the wire passes into the coating bath, where a layer of metal in which abrasive particles are simultaneously embedded is deposited on the wire surface. The abrasive particles have to be incorporated in sufficient quantities over the entire circumference of the wire and homogeneously over the length of the wire. This requires the deposition rate to be kept at a low level. The dwell time of the wire through the apparatus as a whole is determined by this final, time-consuming step. On account of the linear passage of the wire and the low deposition rate in the coating bath, low wire passage rates are required in order to obtain the desired layer thicknesses and concentrations of abrasive particles. After coating has taken place, the wire is wound onto a further reel at the end of the installation. A continuous electrode position coating process of this type is known, for example, from JP-B-4105. The latter document describes a wire passage rate of 60 mm/min and a coating time of 10 minutes. This results in a bath length of 60 cm. Accordingly, it would take approximately 280 hours to coat a wire length of 1000 m. An even lower wire passage rate is required if the coating, as described in EP 128 06 29 A1, is applied using a chemical process. On account of the lower deposition rate caused by process conditions, the passage rate is reduced to approximately 25 mm/min given otherwise identical boundary conditions. Accordingly, in this process it takes approximately 670 hours to coat 1000 m of wire.